Damper Application Guide 1.” 1 Common 2 2 + Hot

Control of Dampers ®®

INDEX

I. INTRODUCTION A. Economizer Systems ...... 3 B. Indoor Air Quality ...... 3 C. Damper Selection ...... 3

II. DAMPER SELECTION A. Opposed and Parallel Blade Dampers ...... 3 B. Flow Characteristics...... 4 C. Damper Authority ...... 4 D. Combined Flow ...... 6

III. ADDITIONAL CONSIDERATIONS A. Economizer Systems ...... 6 B. Building Pressure ...... 8 C. Mixed Air Temperature Control ...... 8 D. Actuators - General ...... 8 E. Hysteresis ...... 9 F. Accuracy ...... 9 G. Actuators - Pneumatic...... 10 H. Positioners ...... 11 I. Torque...... 11 J. Linkage...... 11 K. Actuators - Electronic ...... 12 L. Electronic Actuators - Conventional Crank Arm Type...... 12 M. Electronic Actuators - Direct Coupled ...... 13

IV. OPTIMIZING ECONOMIZER SYSTEMS A. Sizing of Dampers ...... 14 B. Linearization...... 15 C. Range Controller ...... 15 D. Choice of Dampers ...... 16

V. ADVANTAGES A. Direct Coupled Actuators ...... 17 B. Range Controller ...... 17

VI. MODERNIZATION OF OLD INSTALLATIONS ...... 17

VII. SUMMARY ...... 17

REFERENCES

Air Movement and Control Association, Inc. (AMCA) Publication 502-89

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) 1991 Applications Handbook, sec. 41.6

We would like to thank Bengt Carlson, Energy Management Consultant, the primary contributor to this book, along with Larry Felker, Area Sales Manager, and Robert Balkun, Product Manager, for their efforts in creating this Application Guide.

If you have any comments on the information given or possible additions, please send them to Robert Balkun, Product Manager, Belimo Aircontrols (USA), Inc., P.O. Box 2928, Danbury, CT 06813, (800) 543-9038, in CT (203) 791-9915, FAX (203) 791-9919.

The ideas and concepts shown in this book are based on standard, HVAC engineering guide lines. The use of the information given must be used with sound engineering judgement. Belimo Aircontrols does not take responsibility for problems caused by the concepts given in this publication.

All the contents of this publication are the sole property of Belimo Aircontrols (USA), Inc. and may not be reproduced without the written authorization of Belimo Aircontrols (USA), Inc. © 1995 Belimo Aircontrols (USA), Inc.

2 ® Damper Applications Guide 1

I. INTRODUCTION

I-A. Economizer Systems FAN The economizer portion of an air- EA RA handling system is not only responsible for reducing the operating cost of the system, but more importantly, it has to provide a sufficient volume ACTUATORS RA of outside air at all times to ensure good indoor air quality. The func- FAN tion of the economizer also affects Heating/Cooling the pressurization of the building. A OA Etc. SA slight positive pressure is needed to prevent infiltration of unconditioned outside air. Otherwise the indoor climate can be adversely affected due to draft and cold perimeter Fig. 1 - Economizer System walls. This is typically compensated for by increasing the room temperature, which increases the operating cost. However, too high pressure causes exfiltration, which increases the heat losses. Also, an incorrect pressurization may cause damages to the building structure due to condensation, freezing etc. See Fig. 1.

I-B. Indoor Air Quality

The indoor air quality and comfort is much more important than the operating cost of the air handling system. If it is not adequate, absenteeism and turnover will increase and productivity will suffer. This is a hidden cost, which is hard to quantify, but nevertheless, it is a very large cost that can seriously affect the bottom line of any organization. Just imagine what the cost of a few percent decrease in the productivity can be. While the cost of air conditioning typically is $2/sq. ft., the cost of productive workers is $150/sq. ft.

The ventilation system is the cause of almost half the indoor air quality complaints according to NIOSH (National Institute of Occupational Safety and Health). Damper control alone is itself not responsible for the indoor air quality and climate, but it has a very central role. Therefore, it is important that the dampers and actuators are selected, sized and operated correctly.

I-C. Damper Selection

Traditionally, little attention is given to the selection of the dampers that comprise the economizer. What is even worse is that the selection of the actuators that operate the dampers is based upon one criterion only, and that is that they provide sufficient torque to operate the dampers. The accuracy of the control is rarely considered. Without accurate damper positioning, the economizer will not function correctly, and both the indoor air quality and comfort is compromised.

The following text discusses the importance of selecting and sizing the dampers for correct operation.

II. DAMPER SELECTION

II-A. Opposed and Parallel Blade Dampers.

In HVAC installations two different types of rectangular dampers are used to modulate air flow. These are parallel and opposed blade dampers.

Parallel blade dampers are con- structed so all the blades move in the same direction and in parallel. See Fig 2. PARALLEL BLADE OPPOSED BLADE Opposed blade dampers are con- DAMPER DAMPER structed so blades next to each other move in opposite directions. See Fig. 3. Fig. 2 Fig. 3

3 Damper Applications Guide 1 ®

II B. Flow Characteristics NOTE: NOTE: The characteristics will be different when installed The characteristics will be different when installed The two types of dampers have dif- in a system. See Fig. 6. in a system. See Fig. 7. ferent “inherent flow characteristics”. FLOW FLOW This can be illustrated by a curve % % that shows the relationship between 100 100 the flow rate and the position of the 90 90 blades. 80 80 70 70 Fig. 4 shows the inherent flow char- 60 60 acteristics for both parallel and opposed 50 50 blade dampers. Notice that neither 40 40 damper has a linear characteristic. 30 30 20 20 Opposed blade dampers give a very 10 10 slow increase in the flow when the damper begins to open. 0 0

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 °POSITION °POSITION Parallel blade dampers have an inherent curve that is not as pronounced, PARALLEL BLADE DAMPER OPPOSED BLADE DAMPER so the flow increases more rapidly when the damper begins to open. Fig. 4 - Inherent Characteristics of a Parallel and Opposed Blade Dampers It is very important to realize that the inherent curves are measured under laboratory conditions, with a constant CLOSED DAMPER differential pressure across the damper. (In a real installation the F curves will be distorted. See Fig. 6 I C C and 7.) L O O NO FLOW T I I E L L R II-C. Damper Authority

DAMPER PRESSURE DROP = 1.0 IN WG When installed in a system, the damper is not the only device which affects the flow. Other parts of the TOTAL SYSTEM PRESSURE DROP = 1.0 IN WG system, such as the duct work, filters, coils, etc., will also restrict the flow. The result is that the damper Fig. 5A will not control the flow in accordance to its inherent flow characteristics.

When the damper is closed (See Fig. OPEN DAMPER 5A), the full differential pressure will fall across the damper, but when it is F open (See Fig. 5B) the pressure drop I C C L O O LARGE across all other parts will be large, T I I FLOW leaving little pressure drop across E L L R the damper.

The pressure drop across the OPEN DAMPER PRESSURE DROP = .1 IN WG damper changes as the damper is operated from closed to open. This distorts the flow characteristics TOTAL SYSTEM PRESSURE DROP = 1.0 IN WG curve. When the damper begins to open, the differential pressure is high, and the flow increases at a .1 AUTHORITY % = –––– = 10% higher rate than what the inherent 1.0 curve suggests. When the damper is Fig. 5B almost fully open, the pressure drop across the damper will be small,

4 ® Damper Applications Guide 1

compared to the total pressure drop, INSTALLED FLOW CHARACTERISTICS AT so if the damper moves, for example, DIFFERENT DAMPER AUTHORITIES (1-100%) from 80 to 90° opening, the flow will change very little. 100 90 A damper which is sized so it has a 1% small resistance when fully open in 80 2% comparison to the total system resis- 70 3% tance, will have a small influence 4% 5% (authority) on the flow when it oper- 60 10% ates near the fully open position. 15% 50 20% The resistance of the fully open 40 30% damper can be expressed as a per- 50% OF MAXIMUM FLOW 30 centage of the total system resis- 100% tance and is called “Damper 20 Authority” or “Characteristic Ratio”.

PERCENT 10 Damper Authority % = 0 Open Damper Resistance ––––––––––––––––––––– x 100% Total System Resistance 0 10 20 30 40 50 60 70 80 90 DAMPER POSITION, DEGREES OPEN The resistance is a pressure drop, so we can also write: 2 3 4 5 6 7 8 9 10V Open Damper Pressure Drop –––––––––––––––––––––––– x 100% CONTROL SIGNAL Total System Pressure Drop Fig. 6 - Parallel Blade Damper Flow Characteristics The pressure variation across the INSTALLED FLOW CHARACTERISTICS AT damper distorts the inherent charac- DIFFERENT DAMPER AUTHORITIES (1-100%) teristics curves. Instead, we will get an “Installed Damper Characteristic”, 100 which depends upon the Damper 90 Authority. See Fig. 6 and 7. If the authority is 100%, then we will get 80 1% the inherent characteristic, but this 2% 70 3% will rarely happen in a real installa- 4% tion. Instead, a much lower number 60 5% (Ex. 1 - 10%) is realistic. As can be 10% seen in Fig. 6 and 7, at an authority 50 15% of 1%, the curve deviates greatly 20% 40 30% from the inherent curve (100%). 50% OF MAXIMUM FLOW 30 100% It is very important to realize that the 20 “Total System Resistance” or “Total

System Pressure Drop”, only relates PERCENT 10 to the part of the system where the flow is controlled by the damper. 0 See Fig. 8. The flow downstream of 0 10 20 30 40 50 60 70 80 90 the mixing plenum (point A) is not DAMPER POSITION, DEGREES OPEN controlled by the OA damper (it is constant, at least in theory). The OA damper only controls the flow 2 3 4 5 6 7 8 9 10V through the weather louver, the CONTROL SIGNAL damper and the OA duct work. Fig. 7 - Opposed Blade Damper Flow Characteristics Therefore the authority of the OA damper is determined by the pressure drop across the OA damper as a percentage of the total differential pressure between OA and point A.

The total system pressure drop for the OA damper is measured between OA and Point A. The RA damper is measured between Point B and Point A. The EA damper is measured between Point B and EA.

If a single damper is used to control the flow through an air handler and duct work, then the total system pressure drop is the sum of the pressure drops in all the parts. (See Fig. 5A and B)

5 Damper Applications Guide 1 ®

II-D. Combined Flow

EA B RA The sizing of the OA, RA and EA dampers installed in an Economizer is very important because it deter- mines the installed damper charac- RA teristics. If the dampers have been properly sized, their characteristics will complement each other so the ACTUATORS total flow will be constant. See Fig. 9. The pressure at points A and B will remain constant regardless of the OA/RA mixing ratio.

See Fig. 10. The OA damper modulates the OA flow. When it is fully OA A MA SA open, a large portion of the differential pressure will fall across the weather louver and the OA duct Fig. 8 - Economizer System work, resulting in a relatively small differential pressure across the OA damper. When the OA damper is completely closed, the full differential pressure will fall across the OA damper. The authority of the OA FLOW damper is the differential pressure % across the damper when it is fully open, as a percentage of the differ- TOTAL FLOW 100 ential pressure between point A and MA the atmosphere (OA). The calcula- 90 tion of this percentage number is 80 Ideally the combined flow shown as an example in Fig. 10. MA (or SA) should remain 70 RA constant, regardless of the The authority of the RA damper is the differential pressure across the 60 OA/RA mixing ratio. damper, when it is fully open as a 50 percentage of the differential pressure between points A and B. 40 The authority of the EA damper is 30 OA determined by the fully open differential 20 pressure and the differential pressure between point B and the atmosphere. 10 Unfortunately, most dampers are 0 poorly matched. See Fig. 11. This 0 10 20 30 40 50 60 70 80 90 100 % OA/RA MIXING RATIO results in a non-linear OA/RA mixing ratio, a variable combined flow (MA) IDEAL FUNCTION and pressure variations in the mixing plenum, which may affect the building Fig. 9 - Ideal Function pressure.

III. ADDITIONAL CONSIDERATIONS

III-A. Economizer Systems

Most economizer systems have one outside air (OA), one return air (RA) and one exhaust air (EA) damper. The dampers are usually operated in unison, so as the OA and EA dampers close, the RA damper opens. The dampers should be sized so they compli- ment each other, so an increase in the OA flow is matched by an equal decrease in the RA flow. Now, the total flow should be constant, regardless of the mixing ratio between OA and RA flow, and the resistance to the flow will be constant, so the pressure in the

6 ® Damper Applications Guide 1

mixing plenum will stay constant. ∆PT Unfortunately, this is extremely hard ∆PD to accomplish, because the dampers have a non-linear characteristic. FAN There are also other considerations that have to be taken into account, B when the dampers are sized and EA selected. The indoor air quality is very depen-

dent upon the amount of “fresh” out- T P side air that is introduced into the ∆ building. The economizer varies the mixing ratio of outside and return air D RA P to meet the varying conditions. At OA ∆ FAN 100% outside air, maximum free cooling is provided, and when the OA A SA load changes, the mixing ratio is changed until the outside air volume is reduced to a specified minimum ∆PD A = MIXING PLENUM volume which satisfies the indoor quality requirements. In many sys- ∆PT tems, the economizer is at the minimum position, and provides the mini- Pressure drop over damper ∆PD Authority = = ∆ x 100% mum volume of outside air when Total pressure drop PT there is a need for mechanical cool- Example: OA ∆PD = .01 in. W.G. ing or heating. At intermediate load, ∆PT = .125 in. W.G. the mixing ratio is changed to main- .01 Authority = x 100% = 8% tain a constant mixed air (MA) tem- .125 perature. CALCULATION OF THE AUTHORITY

Minimum OA CFM can be set a num- Fig. 10 ber of ways.

A pitot-tube traverse is usually the 150 most accurate if a 100% effective duct length is available to get a 140 MA velocity pressure profile uniform MBINED FLO CO W enough for good readings. 130

120 Totalling outlet CFM is possible but error due to 5 - 15% duct leakage 110 occurs. 100 As a check to one of the above 90 methods, the following formula can be used: TMA = TOA (%OA) + TRA 80

(%RA); where TMA = mixed air temp., OF MAXIMUM FLOW TRA = return air temp., TOA = Outside 70

Air Temp., % OA = % OA volume, % PERCENT 60 RA = % RA volume. If % OA =10%, then % RA = 100-10 = 90%; TOA = 50 60°; TRA = 70°; then TMA = 69° 40 within the error range, ± 2°, of the OPPOSED BLADE RA

10% sensors. It is important to use accu- 1% 30 rate temperature sensors. OA

20 OPPOSED BLADE Some close coupled equipment (rooftop units in particular) are diffi- 10 cult to control properly. The lack of a 0 return fan or exhaust often leads to 0 10 20 30 40 50 60 70 80 90 DAMPER POSITION, DEGREES OPEN OA entering from the so-called EA (OA/RA Mixing Ratio) damper until the RA damper is nearly Fig. 11

7 Damper Applications Guide 1 ®

closed. The building overpressurizes during most of the year.

The minimum outside air volume is accomplished by limiting the modula- EA B tion of the dampers to a minimum position which provides the desired outside air volume. It is very important that this position is controlled accurately because a small positioning error will have a disproportionate- ly large effect upon the minimum out- RA side air volume. An alternative solu- tion is to use two OA dampers. One ACTUATORS is modulated in unison with the RA and EA dampers, as described above. The other damper is smaller and is opened when the fan is running. This damper supplies the mini- Min. OA On/Off mum OA volume. See Fig 12.

DAMPER 2 III-B. Building Pressure

OA A MA SA No building is completely tight. DAMPER 1 Therefore a slight positive pressure is required, in order to prevent any infiltration of outside air into the building. Leakage of air into the building can cause severe problems MODULATING with the indoor climate and the building structure, especially in the heating season. Fig. 12 The pressurizing of a building depends upon a number of factors, and the economizer is just one of them. However, if the economizer system can not maintain a constant pressure in the mixing plenum (Point A in Fig. 12), the pressurization of the building will be affected.

III-C. Mixed Air Temperature Control

The mixed air temperature control is accomplished by varying the mixing ratio between the outside and return air. There are many things that can complicate this function and cause stability problems.

If the dampers are poorly matched, the mixing ratio will not change as a linear function when the dampers are operated. An additional complication is that the pressure in the mixing plenum will not stay constant. The result of this is that at some portions of the damper movement, there will be a large change in the mixed air temperature, while at other portions the change will be quite small. This non-linear function causes stability problems for the mixed air temperature control.

III-D. Actuators - General

When the fans are running, the dampers are subject to forces which increase friction. The resistance created by this friction must be overcome by the actuators. There are also dynamic forces that act upon the damper blades, and tend to turn them either clockwise or counterclockwise, depending upon the position of the blades. The actuators must be able to overcome these forces to position the dampers accurately. See Fig. 17.

Usually, the actuators respond to a variable analog control signal, which either is pneumatic (3…15 psi) or electronic (2…10 V).

Ideally the dampers should be repositioned in direct proportion to the control signal, and follow a “Nominal Signal/Position Curve”. This is usually not the case, because of the hysteresis in the actuators and linkage.

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III-E. Hysteresis ACTUATOR VE POSITION See Fig. 13. All actuators have SIGNAL/ some hysteresis, which means that Hysteresis = 20% % there is one position/signal curve for NOMINAL increasing signals, and a slightly dif- 100 POSITION CUR ferent curve for decreasing signals. C The result is that the position will be 90 Decreasing B slightly different if it is an increasing 80 D or decreasing signal. 70 The control signal has to be changed by a certain value in order to reverse 60 A the movement of the actuator. This Increasing 50 “dead zone” is the hysteresis, and often is expressed as a percentage 40 of the control signal range. Most actuators are connected to the 30 dampers via a linkage, which has an 20 inherent play (slack). The actuator can move back and forth a few per- 10 cent before the damper begins to move. This is also a form of hystere- 0 sis, which adds to the hysteresis of the actuator, so the total hysteresis 0 10 20 30 40 50 60 70 80 90 100 % SIGNAL can be quite large.

The hysteresis creates two serious 3 4 5 6 7 8 9 10 11 12 13 PSI problems: EXAMPLE: Point A The signal is 70%, and the position is 60%. 1. It adds to the stability problems of Point B The signal is increased to 100%, the position is 90%. the mixed air temperature control. Point C The signal is decreased to 80%, the position remains at 90%. 2. It makes the positioning of the Point D The signal is decreased to 70%, the position will be 80%. actuators very inaccurate. This is especially serious with respect to The signal is the same in points A and D, but the position is different, 80% instead of the minimum position of the out- 60%. The resulting actuator position will be different, depending upon whether it is con- side air damper. If the control trolled by an increasing or decreasing control signal. signal is increased from zero (closed OA damper) to the value that represents the Min. position Fig. 13 of the OA damper, a lower than desired Min. position will result. If the control signal is decreased from a high value to the value that represents the Min. position of the OA damper, a higher than desired Min. position will result. This difference in the Min. position, depending upon if it has been approached from above or below, will have a very large impact upon the Min. outside air volume. See Fig. 16.

The hysteresis must not be confused with the “Resolution”. Resolution is the smallest increment the actuator will move, when fine and slow changes in the control signal is made.

The fact that in some applications, actuators can modulate the dampers very smoothly, as long as the control signal is continually changed in the same direction, is not any proof that there is no hysteresis. The hysteresis will only reveal itself when the movement of the damper is reversed. Therefore the hysteresis should be measured by first increasing the signal to, for example, 40%, and then decreasing the signal very carefully, until the damper (not only the actuator) just begins to move. The difference in the signal is the hysteresis, so if the damper begins to close at 25% signal, the hysteresis is 40 - 25 = 15%. Of course, this test must be done with the fans running, applying forces to the damper which the actuator must overcome. Hysteresis should be measured at different points of damper movement, with a special emphasis on the Min. OA position.

III-F. Accuracy

The combination of poorly matched dampers and total hysteresis in the linkage and actuators can make stable control of mixed air temperature very hard to accomplish, even if advanced controllers are used.

If the minimum position of the OA damper is not controlled accurately, the Min. OA volume will be smaller or larger than the required

9 Damper Applications Guide 1 ®

BALL JOINT value. A lower volume will result in a poor indoor air quality. A higher vol- FS FL ume will result in needlessly high SIGNAL operating costs. FA BLADE III-G. Actuators - Pneumatic STEM Pneumatic actuators are so common SPRING SHAFT DIAPHRAGM that we should study their function in FR SIDE SEALS detail.

They are reliable, have spring return FA = FS + FR + FL function, but usually operate with a very high hysteresis. This is inherent FA = Air Signal (Power) Positive LOAD to the design. See Fig. 14. The FS = Spring Positive pressure of the control signal acts on FR = Resistance and Friction Always Resists Change the diaphragm and produces a force, FL = Damper Load Negative and/or Positive which pushes out the stem and com-

For any value of FA (Signal) the actuator takes a position dependent on the forces acting on presses the spring until the forces it. Repeatability is low. The hysteresis is high. With age, FR increases. 1.5 PSI hysteresis are in balance. The position of the is normal over a 5 PSI span. shaft will change in proportion to the control signal. (The nominal sig- Fig. 14 nal/position curve).

However, in order to produce a net 14 + 50 IN-LB force at the output shaft, there must be an imbalance between the 13 0 IN-LB diaphragm and the spring. The force from the diaphragm must be stronger than the spring in order to produce a 12 pushing force, and the force from the – 50 IN-LB VE diaphragm must be less than the

AIR SIGNAL 11 spring force in order to produce a

PSI /POSITION CUR pulling force. This deviation between

10 SIGNAL the position and the nominal signal/position curve is called “spring NOMINALSPRING RANGE 8 - 13 PSI range shift”. 9 8.75 Fig. 15 shows a pneumatic actuator 8 which is required to produce a clockwise (+) 50 in-lb torque to open a 7.25 damper, and a counterclockwise (-) 7 50 in-lb torque to close it. The minimum effective radius of the linkage 0 10 20 30 40 50 60 70 80 90 100% POSITION arm is 2". This means that the pushing or pulling force of the actuator is +25 lb. or - 25 lb. (Torque/Effective PNEUMATIC ACTUATOR radius = Force. 50/2 = 25 lb.) FORCE 7 1/2" OUTSIDE 90° The outside diameter of this actuator DIAM. MIN EFFECTIVE is 7 1/2", which corresponds to an RADIUS = 2" EFFECTIVE DIAPHRAGM effective diaphragm (piston) area of AREA = 33.3 SQ. IN. approximately 33.3 sq. in. REQUIRED TORQUE = 50 in-lb When the signal pressure and the Resultant Pressure Variation to change the direction of the position are in balance, no net force torque from +50 to –50 in-lb: 8.75 - 7.25 = 1.5 PSI Hysteresis = 1.5 PSI is produced at the output shaft. This Hysteresis as a percentage of the spring Range: is represented by the “nominal signal/position” curve. In order to pro- 1.5 ––––––– x 100 = 30% HYSTERESIS =30%! duce a 25 lb. force the signal pres- 13 - 8 sure has to be higher or lower than the nominal signal/position curve. Fig. 15 - Typical Pneumatic Spring Range Shift and Hysteresis

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The resultant pressure variation is 25 lb./33.3 sq. in. = 0.75 psi. Therefore, OA FLOW in order to change from +50 in-lb to % -50 in-lb torque, the total variation has to be 2 x 0.75 = 1.5 psi. This is 30 30% of the spring range of a typical Detail of an Opposed Blade Damper with an authority of 10%* pneumatic actuator (8 to 13 psi).

The hysteresis and spring range shift MIN. 10% OA FLOW DESIRED in an actual installation depends 20 5 - 15% RESULT upon how generously the pneumatic actuator is sized, in comparison to the required torque. If the actuators VARIATION are oversized, the hysteresis will be 50 - 150% OF DESIRED 10 less. The recommendation of the CONTROL POINT = MIN. OA ± 6° OF DESIRED MIN. pneumatic actuator manufacturer’s is VOLUME typically 1 1/4 - 2 in-lb for each OA DAMPER POSITION square foot of damper area. This is less than adequate. The resulting hysteresis can be 12% or more. See 0 Fig. 16. 10 20 30 % OA DAMPER POSITION

III-H. Positioner *If the authority is less, the variation will be even larger. HYSTERESIS = 12% The accuracy of pneumatic actuators is considerably improved if they are Fig. 16 - Results of Hysteresis on Min OA Air Flow provided with positioning relays (“positioner”). A positioner compares the actual position of the actuator with the control signal, and changes the pressure to the diaphragm until the actuator assumes a position that corresponds to the control signal. This reduces the hysteresis to about 1/4 psi, which can be 2 1/2 - 5%, depending upon the operating range. However, frequent recalibration is needed to maintain the accuracy.

The additional cost of the positioner, and its adjustment, must not be forgotten when comparisons are made between pneumatic and electric actuators.

MAXIMUM III-I. Torque 100

The torque that is required to operate 90 a damper depends upon the size, 80 type, quality and condition of the

ORQUE Actual torque needed varies

damper. It is also dependent upon T 70 the differential pressure and air flow. with blade type, pressure Contrary to popular belief, the maxi- 60 drop, system effects, etc. mum required torque is not always at 50 the closed position. Typically, the maximum torque requirement is 40 OF MAXIMUM found at about 30% open position. 30 See Fig. 17. 20

PERCENT 10 III-J. Linkage NEGATIVE POSITIVE 0 The actuator is connected to the damper via a linkage, which has a 0 10 20 30 40 50 60 70 80 90 couple of ball joints, pivots and other DEGREES, DAMPER ROTATION elements that have some play. The slack in the linkage can easily cause Fig. 17 - Typical Torque Requirement a 1 - 3% hysteresis, when the stress changes from a pulling force to a pushing force. This effect requires the actuator to move about 1 - 3% before the damper begins to move. The amount of hysteresis depends upon the condition of the linkage and how well it has been adjusted. If the linkage is improperly adjusted and the joints are worn, the hysteresis can actually be larger than 5%.

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III-K. Actuators - Electronic

Differential Amplifier Electronic proportional actuators + require both a power source, usually Input – 24 VAC, and a control signal, usually Input Signal Signal + 0 to 10 VDC or 4 to 20 mA. Their 2…10 V Conditioner M operation can be compared to a 4…20 mA pneumatic actuator with a positive – positioner. However, unlike its pneumatic counterpart, the electronic actuator requires no field calibration and usually no field maintenance. 90° 10V The position feedback is more pre- cise than a pneumatic device because of a repeatable, geared, interface between the actual actuator Output position and its feedback monitoring Output Signal Signal 0V system. The feedback signal is also 2…10V Conditioner 0° mechanical connection with Gearbox usually available as an output from Feedback the actuator to either monitor the Potentiometer actuator position or as signal for part of a control sequence.

Fig. 18 Feedback can be generated by several methods. Fig. 18 and Fig. 19 show two of these methods. Fig. 18 shows a typical potentiometer circuit used to measure feedback. In this case, the signal from the potentiometer is fed into a differential amplifier along with the input signal. The differential amplifier looks at the difference between the input and feed- Brushless ASIC back signal. It then gives a signal to DC Motor Motor-management move the actuator clockwise or counterclockwise until the feedback signal matches the input signal. Fig. 19 shows a newer method which uses microprocessor technology. In this case, a microprocessor communi- cates to an application specific inte- grated circuit (ASIC). The ASIC both INPUT INPUT SIGNAL MICRO- OUTPUT SIGNAL OUTPUT controls and monitors a brushless SIGNAL CONDITIONER PROCESSOR CONDITIONER SIGNAL 2…10 VDC 2…10 VDC DC motor. By monitoring the digital 4…20 mA pulses generated at the brushless DC motor, the exact position of the actuator can be determined by the microprocessor. The microprocessor also allows for special control criteria to be used in either operational char- Fig. 19 acteristics or input signal processing.

III-L. Electronic Actuators - Conventional Crank Arm Type

Conventional electronic and electrohydraulic actuators typically have a small hysteresis of about 1%. These actuators are mounted to dampers in a similar manner as a pneumatic actuator, by the use of a linkage. This, as in the case of pneumatics, adds 1% to 3% of hysteresis to the system and care must be taken in its setup. Electrohydraulic actuators are not gear type actuators; they work similarly to a pneumatic actuator by building up pressure against a spring. Because they work against a spring they are subject to spring range shift.

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III-M. Electronic Actuators - Direct Coupled

Direct coupled actuators, just as the crank arm type, have a hysteresis of about 1%. However, this style of actuator does not require the use of a linkage. Because of this, the additional hysteresis of a linkage is not present. Without a linkage between the actuator and damper, the damper position can be controlled more accurately and the actuator torque is transmitted more efficiently to the damper. Also, the installation time is dramatically reduced. The following table gives a comparison of several actuator/damper com- binations and their relative accuracy.

DAMPER POSITIONING ACCURACY Pneumatic Pneumatic Electronic Belimo As can be seen, direct coupled elec- Without Positioner With Positioner w/ Linkage Direct Coupled tronic actuators are far more accu- Actuator 12%a 2 1/2 - 5%b 1% 1% rate than any other type of actuator. Linkage 1 - 3% 1 - 3% 1 - 3% None E/P Transducer 2% 2% None None Total 15 - 17% 5 1/2 - 10% 2 - 4% 1% a 30% if undersized. b Before calibration drift

Fig. 20 shows an exploded view of a typical Belimo direct coupled actuator mounting. It is easy to see just Actuator attaches Damper shaft directly to damper how simple and straightforward this shaft with universal mounting method is. Direct coupled mounting clamp. mounting greatly reduces installation time and lowers installation costs because fewer parts are needed. Position indicator

Fig. 21 illustrates the difference Manual override between direct coupled mounting button and mounting using linkage. Notice that the use of linkage often requires Rotation reversing mounting the actuator on an inde- switch pendently located mounting bracket. The linkage itself generally consists Anti-rotation of at least two crankarms connected strap by an operating rod attached to the crank arms with ball joints. Linkage systems can become quite complex Fig. 20 and a number of geometrical issues must be addressed that can effect torque and response times. Please Direct coupled refer to Belimo’s Mounting Methods with universal mounting Guide, Section 6, for more informa- clamp tion about damper linkages.

R It is also possible to mount a direct Crank arm type actuator coupled actuator separately, and 0 1 mounted using linkage connect it to the damper via a link- Belimo age. About 20% of installations direct coupled require this, but it should be used actuator only if it is completely impossible to mount the actuator directly. The Belimo mounting instructions for linkages must be followed. If it is an old installation, replace the old ball joints with new ones that have the least possible play. It is important that the hysteresis be as small as possible.

Fig. 21

13 Damper Applications Guide 1 ®

IV. OPTIMIZING ECONOMIZER SYSTEMS

100 FLOW VARIATION IN A REAL INSTALLATION IV-A. Sizing of Dampers (VARIABLE DIFF. PRESSURE A – B) The installed characteristics are 90 determined by the authority of each

B damper. By sizing each damper cor- 80 EA RA rectly, the installed characteristics of the dampers can be chosen in such 70 a way that they complement each other.

Fig. 22 & 23 show examples of 60 OA MA T O W T A L F L O A dampers that are poorly matched. In Fig. 22 the OA and RA dampers are MA 50 of opposed blade type, and have

OF MAXIMUM FLOW This MA flow variation will only be accomplished high authorities. The result is that under laboratory conditions, when the differential the total air flow (MA) will not be con- 40 pressure across the economizer (between A-B) is held constant. In a real installation, duct work, stant. For example, when the

PERCENT coils, filters, fan curve, etc., will reduce the flow dampers are in the mid position, the 30 variation. Instead the effect will be that the OA = OPPOSED BLADE total flow will be much less than nor- RA = OPPOSED BLADE differential pressure between A-B will vary. Actually, both the flow and pressure will vary. mal. In Fig. 23 the dampers are of 20 20% RA parallel type and have a very small AUTHORITY OA (2%) authority. The result is that the 10 flow, when the dampers are in the AUTHORITY 20% mid position, will be much larger than normal. 0 0 10 20 30 40 50 60 70 80 90 DAMPER POSITION, DEGREES OPEN (OA/RA Mixing Ratio) Fig. 24 shows an example where the dampers have been selected and Fig. 22 - Example of Poorly Matched Dampers sized so their installed characteristics complement each other. The OA CONSTANT DIFFERENTIAL PRESSURE A-B damper has opposed blades, and the This MA flow variation will only be accomplished under laboratory conditions, when the RA damper has parallel blades. differential pressure across the economizer (between A-B) is held constant. In a real Both have an authority of 15%. As installation, duct work, coils, filters, fan curve, etc., will reduce the flow variation. Instead the differential pressure (between A-B) will vary, which will affect the building pressure. can be seen, the total flow (MA) 200 A L F L B remains rather constant regardless T O T O W 190 EA RA of the mixing ratio. As long as the MA 180 variation in the total flow (MA) is less 170 than 15%, the dampers can be 160 regarded as well matched. Of

150 OA MA course, each damper has to be sized A with respect to the air flow and the 140 available differential pressure. The 130 OA damper should always be sized 120 for a larger flow than the EA damper. FLOW VARIATION IN A REAL INSTALLATION 110 (VARIABLE DIFF. PRESSURE A – B) The differential pressure across the 100 RA damper is larger than across the EA damper, so the RA damper will

OF MAXIMUM FLOW 90 be the smallest. 80

70 2% PERCENT Y It is important to remember that there IT A RA R U 60 T OA O H must be a slightly negative pressure H T O U R 50 I at point A, in order for the outside air A T Y % 40 2 to enter the building. The pressure OA = PARALLEL BLADE at point B has to be slightly positive 30 RA = PARALLEL BLADE in order for the exhaust to leave the 20 building. 10 0 Very often the dampers are selected 0 10 20 30 40 50 60 70 80 90 based upon the available duct size, DAMPER POSITION, DEGREES OPEN (OA/RA Mixing Ratio) with little concern for how well they Fig. 23 - Example of Poorly Matched Dampers are matched. In many installations,

14 ® Damper Applications Guide 1

the OA damper is installed next to 120 the weather louver, and therefore OA = OPPOSED RA = PARALLEL has to be the same size to keep the 110 F L O W velocity through the louver below 500 100 T O T A L FPM to avoid snow and rain enter- MA 90 RA B ing. This results in a very small 1 EA RA 80 5% A damper authority and this makes the UT 70 HO sizing of the RA damper very impor- RI TY tant and demanding. 60

OF MAXIMUM FLOW 50 40 OA MA IV-B. Linearization A 30 Y IT PERCENT R O See Fig. 6 & 7. If we study the 20 OA TH AU % installed flow characteristics of a 10 15 damper at different authorities, we 0 will find that the bottom part and top 0 10 20 30 40 50 60 70 80 90 part of the curve is very non-linear. DAMPER POSITION, DEGREES OPEN (OA/RA Mixing Ratio) However, between about 10% and 80% flow, the characteristics are Fig. 24 - Example of Properly Matched Dampers rather linear. This is true for all curves INSTALLED FLOW CHARACTERISTICS AT with authorities between 1% to 50%. DIFFERENT DAMPER AUTHORITIES (1-100%) 100 Most dampers are oversized, so there is no need to open them fully, 90 1% in order to get the desired maximum 80 2% flow. For example, if we limit the 3% 70 4% maximum opening of a damper to 5% 60 80% flow, the non-linear top portion 10% can be eliminated. 50 15% 20% 30% 40 50% Outside air dampers must provide a OF MAXIMUM FLOW specified minimum OA volume. 30 100% Therefore, they must not be modulat- 20 ed below a certain minimum position.

PERCENT 10 This is fortunate, because it elimi- nates the lower non-linear portion of 0 the curve. Now, we can limit the 0 10 20 30 40 50 60 70 80 90 operation of the damper to the linear DAMPER POSITION, DEGREES OPEN portion only. 2 3 4 5 6 7 8 9 10V See Fig. 25, and the 1% authority 2.8V 5.2V curve (opposed blade). This damper CONTROL SIGNAL is operated by a direct coupled actu- Fig. 25 - Installed Opposed Blade Damper Flow Characteristics ator (2 to 10V). In order to open the damper to a 20% (Min. flow), the control signal has to be 2.8 V. In order to open the damper to an 80% flow, the control signal has to be 5.2 V. In other words, it takes only a 2.4 V change in the control signal to get a linear change from Min. to Max. flow. Because only a small portion of the operating range is utilized for this application, the resulting MA temperature can be erratic due to overcontrol.

IV-C. Range Controller

Belimo offers a very useful device, called the SBG24 range controller (see page 18). It accepts the control signal (2 to 10 V) and modifies it so the damper is at the Min. position, when the control signal is 2 V, and the damper is at the Max. position when the control signal is 10 V. In other words, the full swing of the control signal (2 to 10 V) is utilized to vary the flow from Min to Max.

It is common practice to mechanically link the OA and RA dampers. This will save the expense of one actuator. Unfortunately, there are a couple of serious disadvantages associated with this. The most obvious problem is that the linkage will add a hysteresis to the operation of the RA damper. An even more serious problem is that the dampers have to be selected very carefully, so they are well matched. Otherwise the mixing ratio will not be controlled in a linear way, the total flow will not be constant and the pressure will vary. Fig. 22, and Fig. 23, are examples of poorly matched dampers. Fig. 24, is an example of correctly matched dampers.

15 Damper Applications Guide 1 ®

By using the range controller, the operation of the dampers can be EA RA modified so a more linear portion of the dampers are used. This also allows poorly sized dampers to be operated so they can more accurately complement each other. The full ACTUATORS RA range of the control signal is utilized, and there will be a linear relationship between the OA/RA mixing ratio and the control signal. Also, the pressure in the mixing plenum will be con- OA MA SA stant.

See Fig. 26. Each of the dampers * Range given for has its own actuator, so they can be example only. T0 5.2 V (See Fig. 25) operated independently of each 2.8

7.2 to 2.0 V* other. The OA and EA damper actu- Range Range Controller I Controller II ators are connected to the same (Direct Acting) (Reverse Acting) range controller. The RA damper 2…10 V CONTROL SIGNAL actuator is connected to a separate range controller. Fig. 26 It is possible to duplicate the function of the range controller with the software of a DDC system. This is acceptable, but it is not necessarily a cost saving, because an additional analog output is needed. Another problem is that the fine tuning of the range controller should be done in conjunction with the balancing, and it is hard for the balancing contractor to access the DDC controller.

For a detailed description of the Range Controller, see the catalog sheet for SBG 24.

IV-D. Choice of Dampers EA RA Fig. 27 shows an economizer with OA and EA dampers of opposed blade type. The RA damper has parallel blades, mounted so the air flow is deflected up against the OA damper for good mixing. (This is only Parallel Blade one of a number of possible destrati- fication methods.)

RA Because in many installations, the OA damper has the same size as the weather louver, it will be oversized in comparison to the flow, which results OA MA SA in a low authority. Opposed blade dampers have characteristics that can control small air flows more Opposed Blade accurately than parallel blade dampers. For example: in order to Fig. 27 supply a 20% flow, an opposed blade damper, with an authority of 1%, has to open to an 8° position, while a parallel blade damper, with the same authority, should open to a 2.5° position. See Fig. 28 and Fig. 29. If the total hysteresis in the actuator and linkage is just 5°, the parallel blade damper may supply anything from 0% to 46% minimum OA volume (0° to 7.5° damper position). The positioning of an opposed blade damper is not quite as critical. A 5° error will result in a 8 to 30% deviation from the desired OA volume (3° to 13° damper position), so the opposed blade damper is a better choice when the OA damper is oversized and has a small authority. See also Fig. 16. A direct coupled actuator which has a 1% hysteresis will result in only a 2.5% deviation from the desired OA volume.

The range controller has the capability to adapt the control signal so the dampers are operated in such a way that they complement each other. This is done by limiting the damper movement to the linear portion only, and it makes the task of sizing the dampers less

16 ® Damper Applications Guide 1

critical. However, this does not PARALLEL BLADE DAMPER OPPOSED BLADE DAMPER mean that it no longer matters how INSTALLED FLOW CHARACTERISTICS AT INSTALLED FLOW CHARACTERISTICS AT DIFFERENT DAMPER AUTHORITIES (1…100%) DIFFERENT DAMPER AUTHORITIES (1…100%) the dampers are sized. If the 60 60 dampers are correctly sized, the 1% 50 50 range controller only needs to make 3% 1% minor corrections, and a large por- 5% 40 10% 40 tion of the damper movement can be 3% 15% utilized. However, if the dampers are 20% 30 30 5% poorly matched, a larger correction 30% 50% 10% has to be used, and only a small por- OF MAXIMUM FLOW 20 20 20% tion of the damper movement can be 100% OF MAXIMUM FLOW 10 10 50% used. The range controller will allow 8 100% PERCENT a great latitude when sizing the 0 PERCENT 0 dampers, and even if the dampers 0 10 20 30 40 0 10 20 30 40 are poorly matched, the function will 2.5 7.5 DAMPER POSITION 3 8 13 DAMPER POSITION } } be improved. However, the best } } DEGREES OPEN DEGREES OPEN result is achieved if the dampers are –5° +5° –5° +5° not excessively oversized. Hysteresis Hysteresis Fig. 28 Fig. 29 V. ADVANTAGES

V-A. Direct Coupled Actuators

Electronic direct coupled actuators offer a tremendous advantage in that they are very accurate, and can position the dampers with the smallest possible hysteresis. If used in conjunction with the Range Controller, the dampers can be operated at a much superior level of control than would otherwise be possible.

V-B. Range Controller

The Range controller offers the following advantages: • The sizing of the dampers is simplified, because the dampers need not be perfectly matched. • The mixed air temperature control is improved. • The Min. OA position is set in the range controller. • The total flow can be (balanced) adjusted by the range controller. • Because the dampers are operated so they complement each other, the total flow will remain constant regardless of the mixing ratio. • The pressure in the mixing plenum will remain constant. • The range controller can be mounted adjacent to the economizer, so balancing is simplified. Flow measure- ments and adjustments can conveniently be done at the same location.

VI. MODERNIZATION OF OLD INSTALLATIONS

When an old installation is modernized with a DDC system, it can be tempting to keep the old actuators if they still are working. This is always a very unfortunate decision, because of the poor accuracy offered by conventional actuators. The full benefits of the DDC system will not be realized, because the dampers cannot be controlled accurately.

Most existing installations have problematic damper control, which would benefit tremendously if the old actuators were replaced with direct coupled actuators and range controllers, and a recommissioning of the OA CFM is performed.

VII. SUMMARY

This guide has introduced a number of concepts, but is not a complete discussion. The importance of sizing and matching the dampers have been explained, with respect to the authority and other control aspects. However, for the final calculation and sizing of the damper, we refer to the damper manufacturer’s tables, charts and instructions.

The range controller simplifies the sizing, and makes it possible to adjust for the “as built” conditions.

Finally, it cannot be emphasized strongly enough that the accurate and hysteresis free operation of the direct coupled electronic actuators is of the greatest importance for the economical operation of air handling systems. Future guides will discuss variable flow and other systems.

17 Range Controller SBG 24 ®

Application The SBG24 range controller is an interface device used between controller and damper actuator. It is used to compen- sate for the non-linear control characteristics of dampers due to their damper authority.

Operation Wiring diagram The SBG24 is designed to rescale the control output to the

24 VAC Transformer actuator. The maximum and minimum damper position can be 1 set in the SBG24. The controller output signal (2 to 10 VDC, 4 Line ● 1 Common to 20 mA, or 0 to 10 V phasecut) then modulates the actuator Volts ● 2 + Hot I between the maximum and minimum set limits. By only modu- N Control Signal (–) ● 4 lating between these set limits, the damper typically can be 0 to 10 VDC (+) 3 Y1 P controlled more in the linear portion of its damper authority 0 to 20 V Phasecut 4 Y2 U U 2 to 10 VDC 5 U curve. By the use of a SBG24 on both the outdoor air and Feedback signal ● 6 Z T return air dampers in an economizer system, a single con- Override A ● troller output can more accurately position these dampers so Switch Position 3 A = Closed B ● ● ● the change in flow of one damper is off-set by an equal change B = Normal C C = Open 1 Common O in the other. U 2 + Hot T 3 Y1 P Overrides can also be used to make the dampers go to either U a fully open or closed position. 5 U T SBG24 Note: For an explanation of damper authority, see Belimo ”Damper Application Guide 1.” 1 Common 2 2 + Hot

1 Provide overload protection and disconnect as required. 3 Y1 Actuators may be connected in parallel. 2 Power consumption and input impedance must be observed. 2 5 U The actuator and SBG-24 may be pow- 3 ered from the same transformer. …-SR US 5 4 The controller should be powered from a separate transformer.

Wire No. 4 on the NM24-SR US is used for 5 feedback.

Technical Data SBG 24 Power supply 24 VAC ± 20% 50/60 Hz Power consumption 1W without actuator Transformer 1.5 VA without actuator Electrical connection terminals

Control signal Y Y1 0 to 10 VDC Y2 0 to 20 V phasecut Input impedance 100 kΩ (0.1 mA) 8 kΩ (50 mW) Dimensions Operating range 2 to 10 VDC 2 to 10 V phasecut Output signal 2 to 10 VDC (adjustable) max .5 mA Direction of rotation reversible with switch A/B Positioning range adjustable max. = 0.2 to 1 (approx. 20 to 90° rotation angle) min. 0 to 80% of max. Measuring voltage U 2 to 10 VDC (max 0.5 mA) for position 0 to 1 Ambient temperature -4°F to +122°F [-20°C . . . +50°C] Storage temperature -40°F to +176°F [-40°C to +80°C] Housing NEMA type 2 Housing rating UL94V-0 (flammability rating) Quality standard ISO 9001 Weight 14 oz [400 g]